Typically, articular cartilage is a tissue that is not naturally regenerated once damaged. Recently, efforts have been made to reconstruct damaged biological tissues by regenerating a portion of the damaged tissues in laboratories. This approach, defined as “tissue engineering” has raised tremendous attention.
Tissue engineering involves the development of biocompatible materials capable of specifically interacting with biological tissues to produce functional tissue equivalents. Tissue engineering has a basic concept of collecting a desired tissue from a patient, isolating cells from the tissue specimen, proliferating cells, seeding the proliferated cells onto a biodegradable polymeric scaffold, culturing the cells for a predetermined period in vitro, and transplanting back the cell/polymer construct into the patient. After transplantation, the cells in the transplanted scaffold use oxygen and nutrients gained by diffusion of body fluids to proliferate and differentiate to form a new tissue, whereas the scaffold has been dissolved.
The scaffold used for the regeneration of biological tissue is usually comprised of a material that serves as matrix to allow cells to attach to the surface of the material and form a three dimensional tissue. This material should be non-toxic, biocompatible and biodegradable. The most widely used biodegradable polymers, satisfying the aforementioned physical requirements, include organic polymers such as polyglycolic acid (PGA), polylactic-co-glycolic acid (PLGA), poly-ε-caprolactone (PCL), polyamino acids, polyanhydrides, polyorthoesters; natural hydrogels such as collagen, hyaluronic acid, alginate, agarose, chitosan; synthetic hydrogels such as poly(ethylene oxide) (PEO), poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), poly(propylene fumarate-co-ethylene glycol) [P(PF-co-EG) and copolymers thereof.
The aforementioned polymers have been researched to fabricate porous scaffold. However, conventional fabrication techniques generally result in scaffolds with low porosities that do not adequately support cell growth. The pores on the surface of the scaffold are often blocked, nutrients are not sufficiently supplied to the cells, and cells have difficulties in growing into the scaffold. Recently, the application of micro-fabrication technology in the field of tissue engineering has rendered possible the development of complex scaffold with micron-scale resolution. These scaffolds referred to as “microfluidic scaffolds” present a network of micro-channels that allow fluid flow within the scaffold. This network of micro-channels helps to provide both nutrients and soluble factors to distinct sections of the scaffold.
The scaffold can also be encapsulated with a semi-permeable membrane. U.S. Patent Publication No. 2006/0147486 relates to a porous scaffold enveloped with a semi-permeable membrane. This semi-permeable membrane selectively introduces nutrients into the scaffold from outside the scaffold, as well excreting metabolic wastes generated by the tissue cells to the outside of the scaffold. The publication describes the method to grow cells within this scaffold in vitro for regenerating a biological tissue.
U.S. Pat. No. 6,627,422 describes a device containing cells in a yarn matrix encapsulated in a semi-permeable membrane. In this case, the semi-permeable membrane allows implanted cells to receive nutrients but also allows therapeutic molecules produced by the implanted cells to diffuse to host cells. This device is used for cell therapy: the encapsulated cells secrete endogenous proteins to the host. This device functions as a bioartificial organ (for example, as artificial pancreas by secreting insulin).
Despite such progress in the engineering of scaffolds with improved diffusion of nutrients, the scaffold once transplanted to the patient suffers from a limited supply of nutrients. Indeed, in vivo, nutrients and oxygen are delivered to disc cells through blood vessels in the endplates of the vertebrae adjacent to the disc. In degenerative disc disease, the vertebral endplates of vertebrae are not well-functioning and do not allow sufficient diffusion of nutrition to the implanted cell-scaffold.
Different cell types can be used to engineer articular cartilage. Primary differentiated cells of articular cartilage (i.e. chondrocytes) from biopsies of existing cartilage can be used. These cells are often procured from an autologous source since the procurement of heterologous cells or cells from cadavers carries the inherent risk of transfer of pathogens. Mesenchymal Stem Cells (MSCs), which are embryonic-like cells found in bone marrow, are capable of differentiation into different type of mesenchymal tissues and especially cartilaginous tissue; therefore, they are another cell source for cartilage engineering.
However, these cell sourcings raise many issues. Chondrocytes from intervertebral disc are difficult to harvest, because the autologous cells are obtained from the patient's disc and therefore it requires an invasive procedure (back surgery) to perform a biopsy. If cells are harvested from a healthy disc, it jeopardizes the functioning of the healthy disc. If cells are harvested from a damaged disc during the discectomy, it provides abnormal cells from a degenerated tissue. Moreover, chondrocytes are difficult to expand in culture since they de-differentiate. Regarding chondrocytes from other cartilages, the elastic cartilage from the ear is easy to harvest, but it produces only hyaline cartilage and not fibro-cartilage, as in the disc. MSCs also have some disadvantages, because they require a bone marrow biopsy. While a large quantity of cells is needed for tissue engineering, it is difficult to obtain a large quantity of adult stem cells.
Numerous papers have reported the culture conditions that stimulate chondrogenesis of mesenchymal stem cells or de-differentiate chondrocytes. These conditions are the following: high density micromass culture; hypoxia; supplementation with growth factors, such as Bone Morphogenetic Proteins (BMP) particularly BMP-2, -4, -6, and -7, transforming growth factor beta (TGF-β), and/or insulin growth factor one (IGF-I); supplementation with ascorbic acid; culture on specific matrix, such as alginate; culture under mechanical stress such as Intermittent Hydrostatic Pressure (IHP) (Watt, 1988; Dozin et al., 1992; Sullivan et al., 1994; Denker et al., 1999; Zur Nieden et al., 2005; Zhou et al., 2004; Majumdar et al., 2001; Barry et al., 2001; Elder et al., 2005; Mow et al., 1992; Domm et al., 2000).
Few studies have reported the conversion of Human Dermal Fibroblasts (HDFs) into chondrocyte-like cells. U.S. Pat. No. 6,489,165 concerns the conversion of HDFs into chondrocyte-like cells under high density micromass culture and hypoxia. French MM et al. (2004) reported the conversion of HDFs into chondrocyte when the cells are grown on the proteoglycan, aggrecan, and supplemented with insulin growth factor one (IGF-I).
Degenerative Disc Disease
Degenerative Disc Disease (DDD) requires 700,000 procedures each year performed by 4,500 spine surgeons, and the majority of disc disorders occur in young patients. Therefore, it is critical to develop effective and safe strategies to treat this disease.
An intervertebral disc (IVD) is a complex structure comprising three distinctive tissues: the annulus, the nucleus, and cartilage endplates. The annulus is a well-organized, multi-layered structure of collagen fibers. The nucleus is comprised mainly of glycosaminoglycan (hydrophilic polymer). The cartilage endplates supply nutrients. The foregoing combination allows the normal disc to perform two conflicting functions: stability and flexibility.
The intervertebral disc absorbs shocks, maintains motion, and keeps stability. Similar to other cartilages, the innate repair capacity of the intervertebral disc (which acts as a joint between two vertebra) is low, because it is avascular and nutritionally supported only by passive diffusion at the endplates. Consequently, once the degenerative process is activated, it is ultimately considered to be an irreversible condition. Once damaged, the degenerated disc may bulge or extrude, and therefore needs to be removed.
Currently, the common surgical treatment for patients with chronic low back pain due to degenerative disc disease is either discectomy or spinal fusion. Discectomy is an appropriate procedure and is routinely performed to remove the degenerated nucleus through a fenestration within the annulus: it allows removal of both the extruded nucleus (herniectomy) and the degenerated remaining inter-vertebral nucleus fragments. Although this procedure is ideal for decompressing and relieving the nervous system (root or cauda equina), it is a poor operation for the spine, because it creates a potentially disabling condition that leads to a degenerative cascade that may require an additional invasive surgical procedure, like fusion or arthroplasty, for example. Discectomy brings a good short-term effect in relieving radicular pain, but it causes disc height reduction with neuro-foramen stenosis, instability of the treated level, poor result on back pain, and/or complications, such as spinal stenosis or facet pain, for example.
Spinal fusion is the most effective treatment for low back pain. It is a surgical procedure in which an entire disc is removed and the two adjacent vertebrae are united together (“fused”) with the interposition of a graft (cages, bone grafts, and/or fixation devices, for example). It is indicated for patients with advanced disc degeneration. Over 200,000 spinal fusions are performed each year in the U.S. alone, but by eliminating the motion, the spinal fusion alters the biomechanical properties of the inter-vertebral disc and increases stress and strain on the discs that are adjacent to the fused disc. In fact, both discectomy and fusion worsen the condition of the affected disc, adjacent discs, and surrounding tissues (such as facet joints), leading to further degeneration.
The failure of these procedures has led to a search for the development of non-fusion technologies, such as disc or disc nucleus prosthesis, for example. Disc arthroplasty with an artificial disc is an emerging treatment for patients with disc degeneration. Its advantages are to maintain motion, decrease incidence of adjacent segment degeneration, avoid complications related to fusion, and allow early return to function. Today, two kinds of devices are marketed: the total disc replacement and the nuclear replacement, but both of them have major pitfalls. Total disc replacement is a bulky metallic prosthesis designed to replace the entire disc: annulus, nucleus and endplates. These prostheses use an invasive anterior (trans- or retro-peritoneal) approach that requires the presence of a vascular surgeon. Dislodgements, wear debris, degeneration of adjacent intervertebral discs, facet joint arthrosis, and subsidence of this type of prosthesis have been reported. The artificial nucleus substitute preserves the remaining disc tissues and their functions. Its design allows its implantation through a posterior approach, but the major limitation of such nucleus prosthesis is that it can be used only in patients in whom disc degeneration is at an early or intermediate stage, because it requires the presence of a competent natural annulus. Implant extrusion remains a primary concern. As a hydrogel-based device, it is fragile, and so does not resist the outstanding bio-mechanical constraints of the lumbar spine (shear forces). As inert materials, they may lose their mechanical properties over time, and tears and breakages have been reported. Replacing the nucleus only and leaving in place a damaged annulus generates the conditions for implant extrusion or recidivism of discal herniation.
Tissue engineering and regenerative medicine represent a new option for the treatment of DDD. A variety of approaches are used to regenerate tissues. These approaches can be categorized into three groups: 1) biomaterials, without additional cells, that are used to send signals to attract cells and promote regeneration; 2) cells alone may be used, to form a tissue; and 3) cells may be used with a biomaterial scaffold that acts as a frame for developing tissues. While Autologous Chondrocyte Transplantation (ACT) has been used for a few years to repair articular cartilage, tissue engineering for disc repair remains in its infancy. Intensive research is currently done, and animal studies have shown the feasibility of tissue-engineered intervertebral disc. More interestingly, recent pilot clinical studies have shown that ACT is an efficient treatment of herniated disc. The main disadvantage of ACT for disc repair is that it requires a disc biopsy. Therefore, there is a need for an improved method to restore disc anatomy and improve its functioning, and there thus remains a need for an improved method of cartilage repair. The present invention seeks to meet these and other objects and provides a solution to a long-felt need in the art.